Study to reduce the size of particles has been in progress. In particular, intensive study has aimed at reducing particles into nanometer size (for example, in the range of 10 to 100 nm) which is hardly realized by conventional methods of pulverization, precipitation, and others. Further, other study has aimed at not only reducing the size of particles into nanometer size, but also obtaining the particles in a monodispersed condition.
Such nanometer-sized fine particles are different from bulk particles (bigger in size) and from molecules and atoms (smaller in size). That is, the nanometer-size fine particles have a position between them in size. Thus, such nanoparticles are considered to show unexpected properties novel over the conventional size of particles. It is also possible to stabilize the properties of nanoparticles if they can be monodispersed. Thus, nanoparticles having such a possibility are attracting attention in various fields, and they have been studied increasingly in a variety of fields such as biochemistry, new materials, electronic elements, light-emitting display devices, printing, and medicine.
In particular, organic nanoparticles made of an organic compound involve great potential, because the organic compounds, per se, can be modified diversely. Among the organic nanoparticles, organic pigments are used in such applications as painting, a printing ink, an electrophotographic toner, an inkjet ink, and a color filter, and thus the organic pigments are now important materials essential for our everyday life. Particularly, organic pigments are demanded in high-performance with practical importance include pigments for an inkjet ink and a color filter.
Dyes have been used as the colorants for inkjet inks, but pigments are employed recently for solving the problems of dyes in water resistance and light stability. Images obtained by using a pigment ink have an advantage that they are superior in light stability and water resistance to the images formed by using a dye-based ink. However, it is difficult to fine particles uniformly into the nanometer size range (i.e., monodispersing), and therefore the pigment particles can hardly penetrate into the pores on paper surface. As a result, such an image has a problem that the adhesiveness thereof to paper is weaker.
Further, the increase in the number of pixels of a digital camera, there is increased need for reduction in thickness of the color filter for use in optical elements such as an CCD sensor and an display device. Organic pigments have been used in color filters, and a filter thickness depends significantly on the particle diameter of the organic pigment. Hence, it is needed to produce fine particles in a nanometer size, with having stability in a monodispersed state.
As for production methods of organic nanoparticles, studies are made on, for example, a gas-phase method (of sublimating a sample under inert gas atmosphere and depositing particles on a substrate), a liquid-phase method (of obtaining nanoparticles by injecting a sample dissolved in a good solvent through a minute nozzle into a poor solvent of which the agitating condition and the temperature are controlled), and a laser-ablation method (of reducing the size of particles by laser-ablation of a sample dissolved in a solution with laser). There are also reports on preparation of monodispersed nanoparticles having a desired particle size by these methods (JP-T-2002-092700, JP-A-H06-79168, JP-A-2004-91560, and others; “JP-A” means unexamined published Japanese patent application, “JP-T” means published searched patent publication).
On the other hand, there are not many studies on a method of separating and collecting the nanoparticles prepared. In particular, fine particles prepared by the liquid-phase methods or laser-ablation methods are obtained as dispersed in a solvent. Accordingly, it is important to decide how to separate and collect the nanoparticles. Even when desirable nanoparticles are prepared in the form of dispersion, it is not practical, if the particle size changes and the uniformity in the particle diameter is lost in the separation/collection step or if collection of the nanoparticles demands a greater cost.
Although there are disclosed some methods for concentrating and collecting nanoparticles from dispersion, there is still no practical method established, considering industrial-scale production.
For example, JP-A-2004-181312 discloses a method of concentrating nanoparticles by distilling an aqueous nanoparticle-containing solution with an added distillation-accelerating liquid. However, distillation employed in this method demands extra energy such as heating. Thus, this method is not suitable for industrial utilization. Further, since the heat of distillation degenerates the nanoparticles, depending on the kind of nanoparticles, this method limits its application range.
JP-A-2004-292632 discloses a method of adding, to fine particle-containing dispersion, an ionic liquid substantially insoluble therein, and concentrating the fine particles into the ionic liquid. However, the method often results in insufficient concentration of fine particles into the ionic liquid, and thus the method is inefficient.
Further, “Current Pigment Dispersion Technology”, Technical Information Institute Co., Ltd., 1995, p. 166, discloses a method of transferring the pigment and resin from an aqueous phase to an oil phase by using an apparatus called kneader. However, the method disclosed therein is a part of steps for preparing an ink, and it is not certain as to if the method is applicable for concentration of nanoparticles. Besides, the method is unpractical, because it demands a high-strength stirrer, a heating/evacuating step for removal of residual water in the oil phase, and thus a large-scale facility is required for industrial production.
Moreover, separation and collection of nanoparticles, or relevant treatment thereto, involves a problem that the nanoparticles in dispersion liquid may aggregate. As a method of dispersing aggregated nanoparticles in a dispersion liquid, a method of adding a dispersant or an additive, or a mixture appropriately selecting such additives may be possibly considered. However, it is difficult to obtain a sufficiently dispersed state only by addition of additives. Addition of such additives occasionally results in deterioration in the properties of nanoparticles even if the nanoparticles are prepared in a dispersed state. Thus, it is difficult to select a dispersant and other additives that satisfy all requirements.
Other possible dispersion methods include a method of separating particles in the aggregation state by application of physical energy. For example, a method of dispersing particles in ultrasonic cleaning machine are generally described in “Current Pigment Dispersion Technology”, Technical Information Institute Co., Ltd., 1995, p. 166. Alternatively, JP-A-H11-269432 discloses a method of mixing and dispersing functional fine particles in vehicles, preventing aggregation of the fine particles by application of ultrasonic wave, and thus stabilizing the dispersion. However, the method, in which only an ultrasonic wave at a fixed frequency is irradiated, can not satisfy the requirement for further fining and redispersing.
Also disclosed are methods of irradiating ultrasonic waves different in frequency to a pigment dispersed in water containing hydrogen peroxide (JP-A-2003-201419 and JP-A-2004-182751). However, these methods are also not efficient enough in the degree of fine dispersion.